You can get cots that change to toddler beds, then you only have to buy it once.
My twins shared a cot since the day we left hospital. Ofcourse they were tiny so I had one at each end. They stayed sharing the cot until they got too big and the tops of their heads were close to touching (their heads would be in the middle of the mattress as there were one at each end). I didn’t see a point in putting them in separate cots when they were so small. After that I moved one into the other cot which they stayed in until 3 years old when they outgrew it and I then put them into a bunk bed. They shared the bottom bed (top and tail) until they were getting bigger and legs kicking eachother then I let one sleep on the top bed and one on the bottom when they turned 4 years old. They are almost 6 years old now. I’d suggest just single beds would be fine, I only opted for bunk beds to save space but didn’t let them sleep on the top one until I was certain they were safe enough
Our cribs transition into toddler beds and the frame for a twin! They were in the same giant bassinet for the first 6 months and then they started rolling on each other and waking each other up, that's when I separated them and moved them to their room, and they slept so much better. It was harder on me than them